The present invention is directed to an apparatus and method for controlling an automatic transmission for an automobile. More specifically the present invention relates to an apparatus and method for controlling an automatic transmission in the event of an interruption or loss of the vehicle velocity signal.
Conventional automatic transmission control apparatus have a control circuit which gives a shift signal to shift the transmission in response to the vehicle velocity and the output torque of an engine or throttle opening signal. The vehicle velocity signal is supplied from a speed sensor with a reed switch which detects the revolution speed of an output shaft. The speed sensor is comprised of a magnet rotated by the output shaft for a speedometer cable which transmits rotary motion from the output shaft to a speedometer. The reed switch opens and closes under the influence of the rotating magnet to output pules at a frequency related to the velocity of the vehicle. The speed sensor is positioned adjacent to the output shaft or in the speedometer and has its wires connected to the control circuit. With such a speed sensor, the reed switch or the wires may be broken to cause an interruption or loss of the vehicle speed signal. Ordinarily this produces the same effect as a vehicle at rest and the control circuit gives a signal to shift the transmission downwardly to a low gear. This may not be a problem at low vehicle speed but it causes violet braking which jolts the driver and passengers and may lead to engine or transmission failure at higher vehicle speeds.
U.S. Pat. No. 4,499,452 discloses a safety circuit which responds to the interruption or loss of the vehicle velocity signal to control the transmission to maintain a high speed gear ratio. After the vehicle velocity signal is restored, the safety control circuit allows the transmission to change gears. However this safety control circuit maintains the same high speed gear ratio unless the vehicle velocity signal is restored. Therefore usual driving of the vehicle will be impossible once the speed sensor breaks down.